A semiquantitative theory for the 2ƒp radiation observed upstream from the Earth's bow shock

A semiquantitative theory for the 2ƒp radiation observed upstream from the Earth's bow shock is presented: the radiation is produced by the process L + L → T + S, proceeding as two sequential three‐wave steps L → L′ + S and L + L′ → T, in the foreshock where nonthermal L and S waves are observed. (Here L, S, and T denote Langmuir, ion acoustic, and transverse electromagnetic waves, respectively.) This theory is consistent with all the available wave data, including the characteristics and levels of a class of low‐frequency waves identified as S wave products of the process L → L′ + S, and the brightness temperature and bandwidth of the 2ƒp radiation. Indeed the theory could account for higher 2ƒp brightness temperatures if required. Predictions of the theory suitable for observational testing include (1) the existence of two 2ƒp sources, one to each wing of the foreshock, (2) the spatial location of the source regions, and (3) the characteristics and levels of the product L′ and S waves in the source regions. The radiation should (4) have intrinsic bandwidths of the order of 1 kHz or less, (5) be less than 0.1% circularly polarized, and (6) have a limiting brightness temperature equal to the effective temperature TL of the L waves producing the radiation.